The overall objectives of this project are to study the mechanisms underlying the changes in visual sensitivity during light adaptation and dark adaptation. Specifically, we plan to examine in psychophysical experiments whether and under what conditions rod and cone systems contribute independently or interactively to visual sensitivity in humans. We plan to measure early light and dark adaptation to square wave and ramp stimuli. BIBLIOGRAPHIC REFERENCE: Martinez, J.M., Sturr, J.F., and Schmalbach, N.L. The luminance difference threshold as a contrast threshold: evidence for inhibitory interaction in spatial summation. Vision Research, 1977, in press.